1. Field of the Invention
The present invention relates to a novel lubricating oil for compression-type refrigerators. More particularly, the present invention relates to a lubricating oil for compression-type refrigerators which comprises a polyvinyl ether compound having excellent compatibility with hydrogen-containing Flon compounds A "Flon compound" means a chlorofluorocarbon (CFC), a hydrofluorocarbon (HFC) and a hydrochlorofluorocarbon (HCFC) in general.!, such as 1,1,1,2-tetrafluoroethane, difluoromethane, and pentafluoroethane (referred to as Flon 134a, Flon 32, and Flon 125, respectively, hereinafter) and the like, which can be used as the refrigerant to replace compounds causing environmental pollution, such as dichlorofluoroethane (referred to as Flon 12, hereinafter) and the like, as well as with ammonia, exhibiting superior stability and lubricating property, showing low hygroscopicity, and provided with a volume intrinsic resistance of 10.sup.12 .OMEGA..multidot.cm or more at the temperature of 80.degree. C.
2. Description of the Related Arts
Compression-type refrigerators are generally constituted with a compressor, a condenser, an expansion valve and an evaporator and has a structure in which mixed fluid of a refrigerant and a lubricating oil is circulated in the closed system. Temperature is high in the compressor and low in the refrigerating chamber generally in the compression-type refrigerator though the conditions may be different depending on the type of machinery, and it is generally required that the refrigerant and the lubricating oil be circulated in the system without causing phase separation in the wide range of temperature as well as in the wide range of the refrigerant/refrigeration lubricating oil ratio. When phase separation occurs during the operation of the refrigerator, life and efficiency of the apparatus are adversely affected to a great extent. For example, when the phase separation of the refrigerant and the lubricating oil occurs in the part of the compressor, lubrication of the moving parts is deteriorated and seizure occurs to cause decrease in the life of the apparatus to a great extent. When the phase separation occurs in the evaporator, efficiency of heat exchange is decreased because of the presence of lubricating oil of high viscosity.
Because the lubricating oil for refrigerators is used for the purpose of lubricating moving parts in refrigerators, the lubricating property is naturally important. Particularly, because the temperature in the compressor is high, the viscosity which can hold the oil film necessary for the lubrication is important. The required viscosity is different depending on the type of the compressor used and working conditions and it is generally preferable that the viscosity (kinematic viscosity) of the lubricating oil before mixing with the refrigerant be 5 to 1000 cSt at 40.degree. C. When the viscosity is lower than this range, the oil film becomes thin to cause insufficient lubrication. When the viscosity is higher than this range, efficiency of the heat exchange is decreased.
Electric refrigerators have the motor and the compressor built into a single body and the lubricating oil for them is required to have a high degree of electric insulating property. In general, a volume intrinsic resistance of 10.sup.12 .OMEGA..multidot.cm or more at 80.degree. C. is required. When the resistance is lower than this value, possibility of leak of electricity arises.
Furthermore, low hygroscopicity and high stability are required for a lubricating oil. For example, when a lubricating oil has high hygroscopicity, there arises the possibility that water reacts with organic materials to form compounds causing formation of sludge. When organic acids are formed by hydrolysis or the like, corrosion and abrasion of the apparatus tend to take place although degree of the corrosion and the abrasion depends on the amount of the organic acids.
As the refrigerant for compressor-type refrigerators, mainly Flon 12 has heretofore been used and, as the lubricating oil, various types of mineral oils and synthetic oils satisfying the required properties described above have been used. However, Flon 12 is being more rigorously restricted world-wide because it brings environmental pollution such as the ozonosphere destruction. By this reason, hydrogen-containing Flon compounds such as Flon 134a, Flon 32, and Flon 125 are attracting attention as the novel types of the refrigerant. The hydrogen-containing fluorocarbons, particularly Flon 134a, Flon 32, and Flon 125, are preferred as the refrigerant for compression-type refrigerators because they have little possibility of causing the ozonosphere destruction and can replace Flon 12 with little change in the structure of refrigerators which have heretofore been used.
When Flon 134a, Flon 32, Flon 125, or a mixture of these compounds, is adopted as the refrigerant for compression-type refrigerators to replace Flon 12, a lubricating oil having good compatibility with the hydrogen-containing Flon compound, such as Flon 134a, Flon 32, Flon 125, and the like, and good lubricating property satisfying the requirements described above is naturally required. However, because the lubricating oils which have heretofore been used in combination with Flon 12 do not have good compatibility with the hydrogen-containing Flon, such as Flon 134a, Flon 32, Flon 125 and the like, a new lubricating oil suited for these compounds is required. When a new lubricating oil is adopted, it is desired that major change in the structure of the apparatus be not necessary. It is not desirable that the structure of the currently used apparatus must have major changes because of the new lubricating oil.
As the lubricating oil having compatibility with Flon 134a, for example, lubricating oils of polyoxyalkylene glycols have been known. Such lubricating oils are disclosed, for example, in Research Disclosure No. 17463 (October, 1978), the specification of the U.S. Pat. No. 4,755,316, Japanese Patent Application Laid-Open Nos. Heisei 1(1989)-256594, Heisei 1(1989)-259093, Heisei 1(1989)-259094, Heisei 1(1989)-271491, Heisei 2(1990)-43290, Heisei 2(1990)-84491, Heisei 2(1990)-132176 to 132178, Heisei 2(1990)-132179, Heisei 2(1990)-173195, Heisei 2(1990)-180986 to 180987, Heisei 2(1990)-182780 to 182781, Heisei 2(1990)-242888, Heisei 2(1990)-258895, Heisei 2(1990)-269195, Heisei 2(1990)-272097, Heisei 2(1990)-305893, Heisei 3(1991)-28296, Heisei 3(1991)-33193, Heisei 3(1991)-103496 to 103497, Heisei 3(1991)-50297, Heisei 3(1991)-52995, Heisei 3(1991)-70794 to 70795, Heisei 3(1991)-79696, Heisei 3(1991)-106992, Heisei 3(1991)-109492, Heisei 3(1991)-121195, Heisei 3(1991)-205492, Heisei 3(1991)-231992, Heisei 3(1991)-231994, Heisei 4(1992)-15295, Heisei 4(1992)-39394 and Heisei 4(1992)-41591 to 41592. However, the lubricating oils of polyoxyalkylene glycols generally have low volume intrinsic resistances and no example satisfying the value of 10.sup.12 .OMEGA..multidot.cm or more at 80.degree. C. has been disclosed yet.
As the compound having compatibility with Flon 134a in addition to the lubricating oils of polyoxyalkylene glycols, lubricating oils of esters are disclosed in British Patent Laid-Open No. 2216541, WO No. 6979 (1990), Japanese Patent Applications Laid-Open Nos. Heisei 2(1990)-276894, Heisei 3(1991)-128992, Heisei 3(1991)-88892, Heisei 3(1991)-179091, Heisei 3(1991)-252497, Heisei 3(1991)-275799, Heisei 4(1992)-4294, and Heisei 4(1992)-20597 and the specification of the U.S. Pat. No. 5,021,179. However, the lubricating oils of esters inevitably form carboxylic acids because of their structures and the carboxylic acids cause corrosion of apparatuses. For example, rubber hoses are used in air conditioners for automobiles. Lubricating oils of esters cannot be used because moisture may penetrate through the rubber hose. In electric refrigerators, there is no possibility for mixing of moisture during the use. However, the lubricating oil is used for a long time of period without exchange to the new oil and almost all of the moisture present at the time of the initial production is used for hydrolysis to cause problems. Because of these problems described above, modification of the present apparatus or the apparatuses for the production thereof is required to a large extent for using a lubricating of ester in an compression-type refrigerator. Therefore, lubricating oils of esters are not preferable. As a lubricating oil of ester for refrigerators having good resistance to hydrolysis, an oil composition for refrigerators characterized by comprising an epoxy compound is disclosed in Japanese Patent Application Laid-Open No. Heisei 3(1991)-275799. However, the resistance of the oil composition for refrigerators to hydrolysis is exhibited because the epoxy group in the composition is converted to an alcohol by reaction with water. When the content of water is large, there arises the possibility that properties of the oil composition for refrigerators are changed to a large extent by the reaction. Even when the content of water is small, the alcohol formed by the reaction induces transesterification reaction and again there arises the possibility that the oil composition for refrigerators is changed to a large extent. Thus, the oil composition disclosed above is not preferable.
As lubricating oils of carbonates, lubricating oils disclosed in Japanese Patent Application Laid-Open No. Heisei 3(1991)-149295, European Patent No. 421298, and Japanese Patent Application Laid-Open Nos. Heisei 3(1991)-217495, Heisei 3(1991)-247695, Heisei 4(1992)-18490, and Heisei 4(1992)-63893 can be mentioned. However, the lubricating oils of carbonates cannot be free from the problem of the hydrolysis similarly to the lubricating oils of esters.
Thus, it is the real situation at present that a lubricating oil for compression-type refrigerators having excellent compatibility with hydrogen-containing Flon compounds such as Flon 134a, Flon 32, and Flon 125, exhibiting superior stability and lubricating property, showing low hygroscopicity, and provided with a volume intrinsic resistance of 10.sup.12 .OMEGA..multidot.cm or more at the temperature of 80.degree. C. has not been discovered yet. Development of such a lubricating oil has strongly been desired.